1. Field of the Invention
This invention relates to improved electrical feedthroughs, to a method of testing for hermeticity leakage of a hermetically sealed feedthrough, and to electrochemical cells which use the same.
2. Description of the Prior Art
Electrical feedthroughs serve the purpose of providing an electrical circuit path extending from the interior of a hermetically sealed container to an external point which is electrically insulated from the container itself. Many such feedthroughs are known in the art which provide the electrical path and seal the electrical container from liquid and gas and prevent short circuits between the feedthrough ferrule and the lead. A principal difficulty with electrical feedthrough lies in the continuing miniaturization of circuit components, electrochemical cells, and the resulting requirements for correspondingly smaller feedthrough dimensions. As the feedthroughs become miniaturized, it becomes more and more difficult to reliably manufacture a liquid and gas tight electrical feedthrough and to conduct the tests necessary to determine if the feedthrough is performing to specifications.
A particular electrical feedthrough of the present invention is employed in conjunction with an electrochemical cell to provide for an electrical path to the anode or cathode within the cell which is electrically isolated from the cell and the container. Such electrochemical cells are used in implantable medical devices, such as cardiac pacemakers, and in conjunction with complex miniaturized electronic circuits. Electrochemical cells, such as the lithium iodine cell described in U.S. Pat. No. 4,166,158, possess an active cathode which is capable of attacking many seal materials used in electrical feedthroughs, particularly if the seals possess minor cracks or defects. In such instances, the cathode material may both destroy the electrical insulating properties in the glass seal and migrate along the electrical leads to attack the integrated circuits and ultimately may cause the device to cease operating.
Electrical feedthroughs usually include a metal ferrule which retains the electrical lead and seal means. The ferrule is attached, as by welding, to the casing or cover of the electrochemical cell container and has a portion which extends into the interior of the container, and a portion which extends exteriorally of the cover. The interior portions of the feedthrough lead sealing means and ferrule have to be protected from the electrochemically active cathode material as shown, for example, in the aforementioned U.S. Pat. No. 4,166,158, and in the commonly assigned and co-pending U.S. patent application Ser. No. 273,475 filed June 15, 1981, which discloses a feedthrough, the inner portions of which are protected by plastic body which is molded to the feedthrough ferrule and the lead pin to provide for the required insulation in the context of a miniaturized feedthrough assembly.